This is an interesting question to consider. Macbeth's tragic flaw - his overriding ambition - propels him into a world of murder and mayhem, from which he never recovers. However we should also consider if the play suggests that fate, as represented in the witches' prophecy, drives him. However, it seems more likely that the witches merely play on Macbeth's existing inner weakness; his downfall is not the result of any impersonal external agency.
A more important external factor might be said to be his wife, who actively exhorts him to murder. However, as with the witches, she is essentially bringing out what already exists in him - she is not putting a new idea into his head. We might conclude, therefore, that Macbeth's own tragic flaw is indeed the factor that causes his downfall. If he had never been ambitious, he would have surely continued as we see him at the beginning of the play: a noble soldier much honoured and respected by all, rather than being universally reviled, as he is by the end of the play.
In Shakespeare's Macbeth, Macbeth certainly brings about his own downfall with his hubris (his forcing his way above his station in life) and his ambition. But other factors do contribute to his fall.
Macbeth is not the planner in his family--his wife is. He veers from her plan when he kills the grooms, who should be interrogated about what they know of Duncan's death. By veering from his wife's plan he arouses suspicion in Macduff, the hero of the play who will eventually defeat and kill Macbeth. He then shuts his wife out of the decision-making process, and makes his own plans. And his plans are faulty. He replaces his wife as adviser with the witches, who don't have his best interests in mind and use equivocation to trick him. He creates too many coincidences when he kills Banquo and tries to kill Fleance, but Fleance escapes (too many sons killing fathers to be believable). And he really goes over the top, so to speak, when he orders the slaughter of Macduff's family. His fellow thanes and soldiers can only take so much. By the time the battle actually starts, most of Macbeth's subjects have gone over to the opponents.
Also, depending on your point of view and your interpretation, you could argue that fate plays a part in Macbeth's fall. The witches, representing fate in the play, know the future or at least know how to manipulate the present to form the future, and somewhat direct the action. At the least, the witches start the play with a plan and that plan is successful. The witches win, in the play, with no costs to themselves.